This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-252602, filed Aug. 23, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a ball screw apparatus used in a feed mechanism for various equipments or the like.
2. Description of the Related Art
A ball screw apparatus is a mechanical element which can convert rotation movement to linear movement. The ball screw apparatus comprises a screw shaft formed on its outer circumferential face with a spiral ball screw groove, and a nut member fitted on the outer circumferential face of this screw shaft. A ball screw groove opposed to the ball screw groove of the screw shaft is formed on an inner circumferential face of the nut member.
A spiral ball rolling contact way is constituted by the ball screw groove of the screw shaft and the ball screw groove of the nut member opposed to each other. One portion and another portion of the ball rolling contact way which are spaced from each other are connected to each other by a connecting way provided on the nut member. An endless ball circulating way is constituted by the ball rolling contact way and the connecting way. Many balls are received in the ball circulating way in an endless continuous manner.
The nut member is rotatable relative to the screw shaft. For example, when the screw shaft is rotated relative to the nut member, the respective balls positioned between the screw shaft and the nut member circulate in the ball circulating way in an endless manner while they are rotating. Thus, the balls are circulated in the circulating way in the endless manner according to rotation of the screw shaft, so that the nut member can be moved accurately and smoothly relative to the screw shaft in an axial direction of the screw shaft.
FIG. 7 is a cross-sectional view schematically showing a conventional ball screw apparatus 100 of a tube type. A nut member 2 is fitted on an outer circumferential face of a screw shaft 1. A spiral ball screw groove 1a is formed on an outer circumferential face of the screw shaft 1. A ball screw groove 2a is formed on an outer circumferential face of the nut member 2. The ball screw groove 2a is opposed to the ball screw groove 1a of the screw shaft 1. These ball screw grooves 1a and 2a constitute a spiral ball circulating way 3.
Through holes 4a and 4b are formed on a circumferential wall of the nut member 2. These through holes 4a and 4b are connected to each other by a tube 5. The tube 5 is disposed on an outer circumferential portion of the nut member 2. Both end portions of the tube 5 are inserted into the through holes 4a and 4b. A connection way 6 is constituted by the through holes 4a, 4b and the tube 5. The connecting way 6 connects a portion and another portion of the ball rolling contact way 3 spaced from each other.
Thus, both ends of the ball rolling contact way 3 communicates with each other through the connecting way 6 so that an endless ball circulating way 7 is constituted. Many balls 8 are received in the ball circulating way 7. These balls 8 are arranged in the circulating way 7 in an endless manner. Each ball 8 is formed from material selected from steel, ceramic, plastic or the like. The balls 8 are circulated in an endless manner while they are rolling in the circulating way 7 as the screw shaft 1 is rotated.
When the nut member 2 is rotated relative to the screw shaft 1, for example, in a direction indicated with arrow R shown in FIG. 7, each ball 8 is rolled in a direction of arrow R along the screw groove 1a. The moving speed of the ball 8 is slower than the rotating speed of the nut member 2. Therefore, each ball 8 moves relatively in a direction opposed to the arrow R towards the screw groove 2a of the nut member 2 through one through hole 4a from the tube 5.
The conventional ball screw apparatus 100 has a corner portion 10 at a connecting portion between the rolling contact way 3 and the connecting way 6, namely a boundary portion between the screw groove 2a and the connecting way 6, as shown in FIG. 7. For this reason, in the conventional ball screw apparatus 100, there is a case that a smooth circulating movement of the ball 8 circulating in the circulating way 7 is prevented by the corner portion 10, which results in deterioration in operability or durability of the ball screw apparatus 100.
When an external load acts on the ball screw apparatus 100, such a state occurs that a distance between the screw grooves 1a and 2a is reduced or shortened. The reason that the distance between the screw grooves 1a and 2a is because of Hertz""s contact between one screw groove 1a and the ball 8, Hertz""s contact between the other screw groove 2a and the ball 8, and the elastic deformation of the ball 8 itself. In the specification, the state where the distance between the screw grooves 1a and 2a is shortened is referred to as xe2x80x9celastic approachxe2x80x9d.
In the state where the elastic approach occurs, the ball 8 enters from the tube 5 which is a non-loaded zone in between the screw grooves 1a and 2a where is a loaded zone. Here, when the elastic approach occurs, the ball 8 can not enter in between the screw grooves 1a and 2a by its own force. For this reason, a proceeding ball 8 enters in between the screw grooves 1a and 2a due to pushing of the following ball 8.
Conventionally, the ball screw groove 2a is processed continuously over the entire inner circumference of the nut member 2 on the basis of a diameter of circle passing through the center of each ball 8. For this reason, when the above-mentioned external load acts on the ball screw apparatus 100, the screw grooves 1a and 2a mutually approach to each other over the entire inner circumference of the nut member 2 by the same distance. As a result, as soon as the ball 8 has entered in between the screw grooves 1a and 2a from the tube 5, the ball 8 is compressed rapidly.
For example, in a ball screw apparatus of a diameter of 100 mm, a lead of 25 mm, and a ball diameter of 19.05 mm, when a load is applied thereon, the amount of elastic approach between screw grooves reaches 50 xcexcm in some cases. That is, when the ball enters from a non-loaded zone to a loaded zone, compression of 50 xcexcm occurs. For this reason, when a load is significantly large, there may occur such a problem as described below.
(1) Stress is concentrated on the corner portion 10 between the screw groove 2a and the end of the tube 5 so that flaking occurs thereat.
(2) The ball 8 is easily jammed at the vicinity of the corner portion 10, so that the ball 8 is prevented from circulating smoothly.
(3) When the ball 8 enters from the tube 5 into the screw groove 2a, a preceding ball 8 is pushed by the succeeding ball 8 so that the ball 8 is injured or worn. Powders or particles generated by the wear or abrasion cause lowering of durability of the ball screw apparatus.
The present invention is to provide a screw ball apparatus which allows smooth circulation of balls and which can be improved in durability.
A ball screw apparatus of the present invention comprises:
a screw shaft formed on its outer cirmuferential face with a spiral ball screw groove;
a nut member fitted on an outer circumference of the screw shaft and formed on an inner circumferential face with a ball screw groove opposed to the ball screw groove of the screw shaft;
a ball rolling contact way constituted by the respective ball screw grooves opposed to each other;
a connecting way provided in the nut member for causing one portion and another portion of the ball rolling contact way to communicate with each other;
an endless circulating way constituted by the connecting way and the ball rolling contact way; and
a plurality of balls received in the circulating way, and
further comprising a worked portion with a smoothly continuous shape which is formed by grinding a corner portion between the ball rolling contact way and the connecting way.
According to the present invention, balls can circulate smoothly from an initial stage where a ball screw apparatus has just been manufactured. The ball screw apparatus of the present invention has an excellent operability and an enhanced durability. Also, since the balls are facilitated to enter from a non-loaded zone inside the connecting way to a loaded zone between the respective screw grooves, the balls can be prevented from being injured and worn due to contact between the balls.
The worked portion formed by crowning or the like may be formed in a taper shape where the distance between the screw grooves gradually decreases regarding an advancing direction of a ball entering from the connecting way in between the screw grooves. Also, the worked portion may be formed so as to extend from an end of the connecting way in a circumferential direction of the nut member within a range of 90xc2x0 from the corner portion along the circumferential direction of the nut member. With this constitution, when the ball enters from the connecting way into the ball rolling contact way, the ball is gradually pressed so that concentration of stress is prevented occurring in the ball screw groove. Since, when the worked portion extends from the corner portion beyond the range of 90xc2x0 therefrom along the circumferential direction of the nut member, the ball screw apparatus is decreased in loading capacity, the worked portion is limited to at most the range of 90xc2x0 from the corner portion.
It is preferable that the amount of cutting-in in the worked portion is larger than the amount of elastic approach of the ball screw groove. Particularly, regarding the radial direction of the nut member, the cutting-in amount may be set within a range of {fraction (1/400)} to {fraction (1/10)} of the diameter of the ball. When the cutting-in amount is less than {fraction (1/400)} of the ball diameter, the ball is prevented from moving smoothly, which results in failure in solving the above problem. When the cutting-in amount exceeds the {fraction (1/10)} of the ball diameter, the operability of the ball screw apparatus deteriorates.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.